CN101222800A - Control circuit - Google Patents

Abstract

The invention provides a control circuit for controlling current flowing through a load, which comprises a power converter, a controller and a current control circuit. The power converter converts a first voltage signal into a second voltage signal according to a control signal. The controller is coupled to the power converter and adjusts the control signal according to the second voltage signal to maintain the second voltage signal at a predetermined value. The current control circuit is coupled to the load and controls the current flowing through the load according to a dimming signal.

Description

Control circuit

technical field

The invention relates to a control circuit, in particular to a control circuit of a light emitting diode.

Background technique

The dimming circuit is widely used in the brightness control of various display devices such as liquid crystal display panels or light-emitting diodes. It is a common control circuit that uses an external dimming signal to control the current or voltage flowing through the load to produce different brightness. A common light-emitting diode control circuit is to provide a fixed current to determine the brightness of the light-emitting diode. This type of control circuit usually includes a power converter, a controller and a current detection circuit, wherein the power converter includes at least a switching element, a diode, an inductor and an output capacitor, and the controller includes at least A resistor-capacitor (RC) circuit consisting of a capacitor. The controller generates a control signal, and adjusts the control signal by comparing the current detected by the current detection circuit flowing through the LED with a reference voltage. In addition, the switching element in the power converter is turned on or off according to the control signal, and the input supply voltage is converted into an output signal through the diode, inductor and capacitor in the power converter to drive the light emitting diode, and through Adjust the control signal to maintain this current steady. Then, a dimming signal, such as a pulse width modulation signal, is input into the controller, so that the controller adjusts the supply current of the LED according to the dimming signal, so as to achieve the purpose of dimming.

In this type of circuit, since the dimming signal will be affected by the time delay caused by the delay elements (such as switching elements and capacitors) in the controller and power converter, the dimming signal and its reaction to the supply current of the LED will be becomes non-linear. For example, due to the charging and discharging characteristics of the capacitor, the current signal output to the LED will have a rising slope and a falling slope, so even when the switch is turned off by the control signal, the remaining charge in the capacitor will still provide current to the LED. LEDs, so that the LEDs will turn off after a certain period of time, which will cause confusion in use. Therefore, the dimming effect cannot be displayed as expected, and the performance of the dimming circuit is also reduced.

Contents of the invention

In view of this, one of the objects of the present invention is to provide a control circuit for controlling the supply current of the load, so that the current flowing through the load can be changed immediately with the change of the dimming signal without time delay. delay to achieve a better dimming effect.

Based on the above purpose, the present invention provides a control circuit for controlling the current flowing through a load, including a power converter, a controller and a current control circuit. The power converter converts a first voltage signal into a second voltage signal according to a control signal. The controller is coupled to the power converter and the load, and adjusts the control signal according to the second voltage signal to maintain the second voltage signal at a predetermined value. The current control circuit is coupled to the load, and controls the current flowing through the load according to a dimming signal

The present invention further provides a control circuit for controlling the current of at least a first load and a second load. The control circuit includes a power converter, a controller, a first current control circuit, a second current control circuit and a phase shift circuit. Wherein, the power converter converts a first voltage signal into a second voltage signal according to a control signal, and the controller is coupled to the power converter, and adjusts the control signal according to the second voltage signal so that the second voltage signal maintains a predetermined value. The first current control circuit is coupled to the first load for controlling the current flowing through the first load, the second current control circuit is coupled to the second load for controlling the current flowing through the second load, and the phase shift circuit Used to generate a first control signal and a second control signal with different phases according to a dimming signal, wherein the first control signal is used to control the first current control circuit, and the second control signal is used to control the second current The control circuit controls the current flowing through the first load and the second load respectively.

Moreover, the present invention further provides a control circuit for controlling the current flowing through at least a first load and a second load. The control circuit includes a power converter, a controller, a first current control circuit, a second current control circuit and a phase shift circuit. Wherein, the power converter converts a first voltage signal into a second voltage signal according to a control signal, and the controller is coupled to the power converter, and adjusts the control signal according to the second voltage signal so that the second voltage signal maintains a predetermined value. A first and a second current control circuit are respectively coupled to the first load and the second load for controlling the current flowing through the first load and the second load and the phase shift circuit for generating and dimming according to a dimming signal The signals have at least one control signal with different phases, wherein the first current control circuit is based on the dimming signal, and the second current control circuit is based on the control signal to respectively control the current flowing through the first load and the second load.

In order to make the above and other objects, features, and advantages of the present invention more comprehensible, preferred embodiments are listed below and described in detail in conjunction with the accompanying drawings.

Description of drawings

FIG. 1 shows an embodiment of a load control circuit.

FIG. 2 is a schematic block diagram showing a load control circuit according to an embodiment of the present invention.

FIG. 3 is a detailed circuit diagram showing a load control circuit according to an embodiment of the present invention.

FIG. 4 is a schematic diagram showing another load control circuit according to an embodiment of the present invention.

FIG. 5 is a schematic block diagram showing another load control circuit according to an embodiment of the present invention.

FIG. 6 is a schematic block diagram showing yet another load control circuit according to an embodiment of the present invention.

FIG. 7 is a schematic diagram showing a dimming signal according to an embodiment of the present invention.

FIG. 8 is a schematic block diagram showing yet another load control circuit according to an embodiment of the present invention.

Description of reference symbols

100-load control circuit; SW-switching element; VDD-input voltage signal; L-inductance;

C-output capacitor; D-rectifier diode; 200-schematic diagram; 210-power converter;

220-controller; 230-current control circuit; 240-load; OUT-output voltage signal;

S1-dimming signal; S2-control signal; 300-load control circuit;

310, 310'-power converter; 320-controller; 330-current control circuit;

340-light-emitting diode module; V _REF -reference voltage; 322-error amplifier; 324-comparator

326-resistance-capacitance (RC) circuit; 328-triangular wave generating unit; 329-drive circuit;

M1, M2-switching elements; 500, 600-schematic diagram; 610-power conversion; 620-controller;

630-phase shift circuit; 640, 650-current control circuit; 660, 670-load; t1, t2-time;

S11, S12-control signals.

Detailed ways

The present invention is a control circuit for controlling the supply current of a load such as a light-emitting diode, and receiving a dimming signal so that the current flowing through the load can be changed immediately with the change of the dimming signal without causing time lag Delay to achieve better dimming effect. To achieve this purpose, the present invention uses the voltage of the feedback (feedback) load as the main feedback signal to maintain the stability of the supply voltage, and the dimming signal is directly input to a current control circuit composed of a current mirror circuit to directly The purpose of dimming is achieved by controlling the current flowing through the load, and the current controlled by the current control circuit will not be fed back to the circuit including the aforementioned delay elements (such as switching elements, capacitors, etc.). According to the present invention, since the dimming signal does not pass through the aforementioned delay element, it is not affected by its time delay, so that the linearity of its response to the supply current of the LED is better. In particular, when the dimming signal is at a low level, the supply current of the load can be quickly reduced to zero, so that the load can be turned off immediately with the low level of the dimming signal, achieving a better control effect.

In addition, the present invention further provides a control circuit including a phase shift circuit, which can be used in a multi-load circuit with more than two loads, so that it can use the dimming signal to generate multiple control signals with different phases to control each current control circuit to drive each load.

Please refer to FIG. 1 , which shows a preferred embodiment of a control circuit. The control circuit 100 here is the circuit shown in FIG. 1 minus the load, that is to say, the control circuit 100 does not include a load. It can be seen from FIG. 1 that the current of the load (a light emitting diode in this embodiment) in the figure is detected by the control circuit 100, and compared with a reference voltage to adjust a control signal, and control the switching element SW through the driving circuit. The input supply voltage VDD is converted into an output voltage signal through the rectifier diode, the inductor L and the output capacitor C, so as to provide a current for the load. Wherein, an RC circuit is used for feedback compensation control, and a comparator is compared with a triangle wave to adjust the pulse width of the control signal to maintain a stable supply current to the load. The dimming signal is coupled with the detection current of the load to control the magnitude of the current flowing through the load to achieve the purpose of dimming.

However, in the control circuit 100, since the dimming signal can at least reflect the output signal through the loop of the RC circuit, the switching element SW, and the output capacitor C, the capacitor in the RC circuit, the switching element SW, and the output capacitor C cause The delay characteristic makes it impossible for the output signal to the load to change immediately with the dimming signal. For example, if the dimming signal is at a high level, although the switching element SW is turned off, the output capacitor C still retains residual charges to provide to the load, so the load will not be turned off immediately following the dimming signal. After a period of time, it will not be turned off until the output capacitor C is discharged to a certain extent. Such a delay phenomenon will make the dimming effect not good.

Please refer to FIG. 2 which shows a block diagram 200 of a control circuit according to an embodiment of the present invention. As shown in the figure, the control circuit of this embodiment includes a power converter 210 , a controller 220 , and a current control circuit 230 for controlling a load 240 . Please note that the control circuit of the present invention is the dotted area shown in the figure, that is, it is composed of the power converter 210 , the controller 220 and the current control circuit 230 , and does not include the load 240 . The load in the figure is only used to assist in explaining the operation of the control circuit and the relationship with the load. To simplify the illustration, the load parts in the following figures are not included in the control circuit of the present invention.

Wherein, the controller 220 generates a control signal S2, and the control signal S2 may be a pulse width modulation signal. The power converter 210 is coupled to the controller 220 and the load 240, and converts the input supply voltage VDD into an output voltage signal OUT to the load 240 according to the control signal S2 generated by the controller 220, and generates a feedback signal to the load 240 according to the output voltage signal OUT. controller 220 . Therefore, the controller 220 can adjust the pulse width of the control signal S2 according to the feedback signal, so that the power converter 210 can generate a stable voltage output.

In this embodiment, the current control circuit 230 is coupled to the load 240 , and controls the magnitude of the current flowing through the load 240 according to the level of a received dimming signal S1 . When the dimming signal S1 is at a high level, the current control circuit 230 can output a current to drive the load 240 . At this time, the output voltage signal OUT maintains a predetermined value under the control of the controller 220 . When the dimming signal S1 is at a low level, the current control circuit 230 does not output current to the load 240 , that is, the current flowing through the load 240 is zero, so that the load 240 stops operating immediately. At this time, since the controller 220 uses the feedback output voltage signal OUT as the main source of feedback control, although the current flowing through the load is zero, it will not affect the controller 220 and the output voltage signal OUT, so the output voltage signal OUT remains a predetermined value. That is to say, with such a circuit configuration, the load 240 can obtain a fixed supply voltage without being affected by the dimming signal S1. The dimming signal S1 in the above embodiment is a pulse width modulation signal, but the present invention can also be implemented using a DC dimming signal.

FIG. 3 shows a detailed circuit diagram of a control circuit 300 according to an embodiment of the present invention. The control circuit 300 includes a power converter 310 , a controller 320 , and a current control circuit 330 for controlling an LED module 340 . Wherein, the power converter 310 is coupled to the controller 320 and the LED module 340 , and the current control circuit 330 is coupled to the LED module 340 . In addition, the dimming signal S1 is input into the current control circuit 330 . Please note that although in this embodiment, the power converter 310 is a step-down power converter (buck converter), the load is a light emitting diode module 340, and the dimming signal S1 is a pulse width modulation signal, these are only used for It is not intended to limit the present invention to this.

The power converter 310 includes at least a rectifying element (such as a rectifying diode D), an inductor L, an output capacitor C, and a switching element SW. Wherein, the switch element SW has a first input terminal, a second input terminal and a control terminal, and the first input terminal receives an input voltage signal VDD. The cathode terminal of the rectifier diode D is coupled to the second input terminal of the switch element SW, the anode terminal of the rectifier diode is grounded, one end of the inductor is coupled to the cathode end of the rectifier element, and one end of the capacitor is coupled to the other end of the inductor. During operation, the switching element SW receives a control signal S2 generated by the controller 320, switches the input voltage signal VDD into a square wave, and converts the input voltage signal VDD into an output signal OUT through the rectifier diode D, the inductor L and the capacitor C. .

In this embodiment, the controller 320 includes a reference voltage V _REF , an error amplifier (erroramplifier) 322, a comparator 324, a feedback compensation circuit (such as a resistor-capacitor (RC) circuit 326), a signal generator unit (eg triangular wave generating unit 328 ) and a driving circuit 329 . The error amplifier 322 has a forward terminal and an inverting terminal. The inverting terminal receives a feedback signal. The feedback signal is generated according to the voltage output signal OUT output by the power converter 310. The forward terminal is coupled to the reference voltage V _REF catch. The error amplifier 322 compares the feedback signal received at the inverting terminal with the reference voltage V _REF at the forward terminal, and performs signal compensation and feedback control through the RC circuit 326 to output an adjustment signal.

The comparator 324 has a forward end and an inverting end, the forward end receives the adjustment signal sent by the error amplifier 322 , and the inverting end receives a triangular wave signal generated by the triangular wave generating unit 328 . The comparator 324 adjusts the pulse width of the control signal S2 by comparing the adjustment signal with the triangular wave signal. The driving circuit 329 controls the switching element SW in the power converter 310 to be turned on or off according to the control signal S2 , so that the power converter 310 generates the output voltage signal OUT. Wherein, the signal generation unit can also be a sawtooth wave generation unit (not shown), which is used to generate a comparison signal of the sawtooth wave signal.

The current control circuit 330 includes at least two switch elements M1 and M2 , which are coupled to the dimming signal S1 , that is, the dimming signal S1 is input into the current control circuit 330 . In this embodiment, the current control circuit 330 is a current mirror circuit. It should be noted that, in addition to the current mirror circuit, the current control circuit 330 can also be any current regulation circuit that can regulate the current.

Please refer to FIG. 3 , since the dimming signal S1 is coupled to the current control circuit 330, when the dimming signal S1 is at a high level, the switching elements M1 and M2 in the current control circuit 330 will be turned on, and the switching element M1 terminal The current will cause the switch element M2 to generate proportional current, so the current flowing through the LED module 340 can be controlled to drive the LED module 340 to emit light according to the high level of the dimming signal S1. When the dimming signal S1 is at a low level, the switching elements M1 and M2 in the current control circuit 330 will be turned off at the same time, so that the negative terminal of the LED module 340 is in an open state, although the output of the power converter 310 The capacitor C still has charge, but because the negative end of the LED module 340 is open, there is no discharge path, so no current is output to the LED module 340, resulting in no current output to the LED module 340, so that the LED module 340 With the low level of the dimming signal S1, it is turned off immediately.

For example, assuming that the dimming signal S1 is a pulse width modulation signal with a high level of 2V and a low level of 0V, when the dimming signal S1 is 2V, the LED module 340 will receive, for example, a 20mA , to emit light, when the dimming signal S1 is 0 volts, the current flowing through the LED module 340 will immediately be zero and the LED module 340 will be turned off. Therefore, the current flowing through the LED module 340 can change with the dimming signal S1 without being affected by the delay effect of the delay element, so that there is a better relationship between the current flowing through the LED module 340 and the dimming signal S1. linearity. It should be noted that, in addition to the delay effect caused by the capacitor C in the power converter 310, the control circuit also includes delays that may be caused by the RC circuit 326 in the controller 320 and the switching element SW in the power converter 310. Effect, among them, the delay effect of capacitor C is much greater than other delay effects. Regardless of the delay caused by any of the above elements, the present invention can avoid its delay effect.

In addition, when the working voltage required by the LED module 340 is greater than the input voltage, the above-mentioned power converter 310 can also be replaced by a boost converter 310', as shown in FIG. 4 . FIG. 4 is a schematic diagram of another control circuit according to an embodiment of the present invention. Wherein, the power converter 310' is a boost power converter. The current control circuit of the present invention is used to control whether the load current is turned on or not, regardless of whether the power converter is a boost type or a buck type. Therefore, referring to the description of the aforementioned control circuit 300 , it can be known that the circuit configuration in this embodiment will also enable better linearity between the current flowing through the LED module 340 and the dimming signal S1 .

FIG. 5 shows a schematic block diagram 500 of another control circuit according to an embodiment of the present invention. Please refer to FIG. 2 and FIG. 5 at the same time. The block diagram 500 in FIG. 5 is different from the block diagram 200 in FIG. 2 in that the dimming signal S1 and the current control circuit are coupled to the high voltage end of the load instead of the low voltage end. The rest of the configuration is the same, and its working principle is also the same. As long as the structure of the current control circuit is properly selected, the current control circuit can also be used to make the current flowing through the load not be delayed by the delay element (output capacitor C) in the power converter, and can follow the dimming signal Changes with changes in S1. Therefore, the current control circuit of the present invention can be placed at any end of the load (high voltage end or low voltage end).

In addition, for a circuit with multiple loads, the simultaneous conduction and open circuit of each load current will cause a huge ripple on the current. In order to reduce the above-mentioned current ripple, the present invention uses a phase shift circuit to generate control signals of different phases to each current control circuit, and then drives each load to make it turn on and open with a time difference to reduce the size of the ripple .

FIG. 6 shows a schematic block diagram 600 of another control circuit according to an embodiment of the present invention. As shown in FIG. 6 , the control circuit of this embodiment includes a power converter 610 , a controller 620 , a phase shift circuit 630 , a current control circuit 640 and a current control circuit 650 . The controller 620 generates a control signal, and the current control circuits 640 and 650 are respectively used to control the current flowing through the loads 660 and 670 to drive or turn off the loads 660 and 670 . The power converter 610 is coupled to the controller 620 and the loads 660 and 670, and converts an input voltage VDD into an output voltage signal according to a control signal. The controller 620 is coupled to the power converter 610 and adjusts the control signal according to the output voltage signal to maintain the output voltage signal at a predetermined value. Please note that the detailed circuits of the power converter 610, the controller 620, and the current control circuits 640 and 650 in the block diagram 600 are similar to the structure and structure of the power converter 310, the controller 320, and the current control circuit 330 in the control circuit 300. For the configuration method, please refer to the relevant description of the control circuit 300 , and the details are not repeated here.

The phase shift circuit 630 is coupled to the current control circuits 640 and 650 for generating a first control signal S11 and a second control signal S12 with different phases according to a dimming signal S1, wherein the first control signal S11 is used To control the current control circuit 640 , the second control signal S12 is used to control the current control circuit 650 to control the current flowing through the load 660 and the load 670 respectively. FIG. 7 shows a schematic diagram of a dimming signal according to an embodiment of the present invention. As shown in the figure, the phase shift circuit 630 generates a first control signal S11 at time t1 according to the original dimming signal S1, and then generates a second control signal S12 at time t2. Therefore, the first control signal S11 has a different phase from the second control signal S12. The first control signal S11 is used to control the current control circuit 640 to generate a corresponding current to drive the load 660 , and the second control signal S12 is used to control the current control circuit 650 to generate a corresponding current to drive the load 670 . Since the load 660 and the load 670 work in different phases, the output current ripple of the power converter can be reduced, and the performance of the power converter can also be improved. It should be noted that in this embodiment, although the original dimming signal S1 has a different phase from the first control signal S11, in other embodiments, the original dimming signal S1 can also have the same phase as the first control signal S11. . Furthermore, when there are more loads to be controlled, according to the present invention, the phase shift circuit can be used to generate corresponding control signals with different phases to drive each load to achieve the purpose of dimming.

In addition, the dimming signal S1 can also be directly input to one of the current control circuits, and the phase shift circuit generates a control signal to control other current control circuits. FIG. 8 shows a schematic block diagram of yet another control circuit according to an embodiment of the present invention. The control circuit has a first load and a second load. As shown in the figure, the controller generates a control signal, and the power converter is used to convert an input voltage VDD into an output voltage signal. The controller is coupled to the power converter, and adjusts a control signal according to the output voltage signal to adjust the output voltage. The level of the signal keeps the output voltage signal at a predetermined value. The current control circuit is used for controlling the current flowing through a second load. The phase shift circuit is used to generate a control signal having a phase different from that of the dimming signal according to a dimming signal S1, wherein the first current control circuit is directly controlled by the dimming signal, and the second current control circuit is controlled by the control signal, so that The currents flowing through the first load and the second load are respectively controlled. Wherein, the first and second current control circuits can be current mirror circuits, and the power converter can be a buck power converter or a boost power converter. The detailed circuits of the power converter, the controller and the first and second current control circuits are the same as the structure and configuration of the power converter 310, the controller 320 and the current control circuit 330 in the load control circuit 300, please refer to the load control circuit 300 The relevant instructions, the details will not be repeated here.

When the above-mentioned multi-phase dimming control circuit is applied to the backlight module of liquid crystal, it can cooperate with scanning backlight (Scan Backlight) and other technologies (that is, multiple light-emitting modules in the backlight module emit light sequentially at different time points, thereby improving the afterimage phenomenon. ), that is, the phase shift circuit receives the vertical scanning signal Hsync of the liquid crystal panel, and generates a plurality of synchronous signals of different phases to each current control circuit, so as to achieve the function of reducing the afterimage of the liquid crystal.

The above description provides several different embodiments or different ways of applying the invention. The specific devices and methods in the examples are used to help explain the main spirit and purpose of the present invention, but of course the present invention is not limited thereto.

Therefore, although the present invention has been disclosed as above with preferred embodiments, it is not intended to limit the present invention. Any person familiar with the art may make some changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be defined by the patent scope of the present application.

Claims (14)

1. A control circuit for controlling the current flowing through a load, comprising: A power converter, converting a first voltage signal into a second voltage signal according to a control signal; a controller, coupled to the power converter, and adjusts the control signal according to the second voltage signal so that the second voltage signal is maintained at a predetermined value; and A current control circuit is coupled to the load and controls the current flowing through the load according to a dimming signal. 2. The control circuit as claimed in claim 1, wherein the power converter comprises: A switch, which has a first input terminal, a second input terminal and a control terminal, the first input terminal receives the first voltage signal; a rectifying element, one end of which is coupled to the second input end of the switch; an inductor, one end of which is coupled to the end of the rectifying element; and A capacitor, one end of which is coupled to the other end of the inductor, wherein the control end of the switch receives the control signal, and converts the first voltage signal into the first voltage signal through the rectifying element, the inductor, and the capacitor. Two voltage signals. 3. The control circuit as claimed in claim 1, wherein the controller comprises: A feedback compensation circuit having at least one capacitor; An error amplifier, which has a forward end and an inverting end, the inverting end receives the second voltage signal, the forward end is coupled to a reference voltage, wherein the error amplifier compares the inverting end with receiving the second voltage signal and the reference voltage at the forward end, and outputting an adjustment signal through feedback control of the feedback compensation circuit; a signal generating unit, used to generate a comparison signal; A comparator has a forward terminal and a reverse terminal, the forward terminal of the comparator receives the adjustment signal, and the reverse terminal receives the comparison signal to generate the control signal; and A driving circuit, coupled to the comparator, utilizes the control signal to control the switch of the power converter. 4. The control circuit as claimed in claim 1, wherein the comparison signal is a triangular wave signal or a sawtooth wave signal. 5. The control circuit as claimed in claim 1, wherein the power converter is a buck power converter or a boost power converter. 6. The control circuit as claimed in claim 1, wherein, when the dimming signal is a low-level signal, the current control circuit immediately makes the current flowing through the load zero. 7. The control circuit as claimed in claim 1, wherein the current control circuit is a current mirror circuit. 8. The control circuit as claimed in claim 1, wherein the dimming signal is a pulse width modulation signal or a DC signal. 9. The control circuit of claim 1, wherein the load is a light emitting diode. 10. A control circuit for controlling current flowing through at least a first load and a second load, comprising: A power converter, converting a first voltage signal into a second voltage signal according to a control signal; a controller, coupled to the power converter, and adjusts the control signal according to the second voltage signal, so that the second voltage signal maintains a predetermined value; a first current control circuit, coupled to the first load, for controlling the current flowing through the first load; a second current control circuit, coupled to the second load, for controlling the current flowing through the second load; and a phase shift circuit, used to generate a first control signal and a second control signal with different phases according to a dimming signal, Wherein, the first control signal is used to control the first current control circuit, and the second control signal is used to control the second current control circuit, so as to respectively control the current flowing through the first load and the second load. 11. The control circuit according to claim 10, wherein when the dimming signal is a low-level signal, the first current control circuit and the second current control circuit make the current flow through the first load and the second current control circuit The current of the second load is zero immediately. 12. The control circuit according to claim 10, wherein the first load and the second load are light-emitting modules in a backlight module of a liquid crystal panel, and the first control signal generated by the phase shift circuit and the liquid crystal A vertical scanning signal of the panel is synchronized. 13. The control circuit according to claim 10, wherein the currents flowing through the first load and the second load are respectively controlled by the first current control circuit and the second current control circuit without feedback to the power conversion device and the controller. 14. A control circuit for controlling current flowing through at least a first load and a second load, comprising: A power converter, converting a first voltage signal into a second voltage signal according to a control signal; a controller, coupled to the power converter, and adjusts the control signal according to the second voltage signal, so that the second voltage signal maintains a predetermined value; a first and a second current control circuit, respectively coupled to the first and the second load, for controlling the current flowing through the first and the second load; and a phase shifting circuit for generating at least one control signal having a phase different from that of a dimming signal according to a dimming signal, Wherein, the first current control circuit controls the current flowing through the first load and the second load respectively according to the dimming signal, and the second current control circuit according to the control signal.

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